nthroot of a sum of surds

    added ``minimal_polynomial_sq`` and ``nthroot``
    
    ``minimal_polynomial_sq`` returns the minimal polynomial for the
    ``nth-root`` of a sum of surds.
    
    ``nthroot`` computes a real ``nth-root`` of a sum of surds.

    modified ``minimal_polynomial`` to call ``minimal_polynomial_sq``;
    solved issue 2895